Preceding stimulus frequency-dependent potentiation of the postrest shortening of the action potential duration in rabbits

Action potential duration (APD) in rabbit ventricular myocardium shortens a fter a rest period (postrest shortening). However, the effects of preceding stimulus frequency on the postrest shortening have not been elucidated. We recorded transmembrane action potentials (TAPs) and monophasic action pote ntials (MAPs) from the rabbit ventricle. In in vitro experiments, repetitiv e regular stimuli (S1) at cycle lengths ranging between 500 to 3000 ms were followed by a single extrastimulus (S2) at a coupling interval of 5000 ms. A decrease in S1S1 interval resulted in a progressive shortening of the du ration of TAP (TAPD) elicited by S2 (S2-TAPD), which was potentiated by inc reasing extracellular calcium concentration ([Ca2+](o)) or application of o uabain and was inhibited by lowering [Ca2+](o) or verapamil, Application of ryanodine was most effective in lengthening S2-TAPD following a short S1S1 interval. 4-aminopyridine and E4031 caused marked lengthening of S2-TAPD w hen S1S1 was long. However, the lengthening effect was attenuated and disap peared with a shorter S1S1 interval. In in vivo experiments, regular ventri cular pacing (S1) at cycle lengths ranging between 250 to 1000 ms was follo wed by a single extrastimulus (S2) with a coupling interval (S1S2) of 1500 ms. A decrease in the S1S1 interval also resulted in progressive shortening of the duration of MAP elicited by S2. Our results indicate that the postr est shortening is potentiated by an increase in the preceding stimulus freq uency in the rabbit ventricle, in which the function of sarcoplasmic reticu lum may play a significant role.